The invention relates to miniature transformers that are useful in hybrid integrated circuits, and more particularly to planar transformers having primary and secondary spiral windings imprinted on a circuit board, with a ferrite core formed by two ferrite sections, each including a thin, flat plate and separated from the other by two post sections and also supported by the circuit board.
Hybrid electronic integrated circuits often are constructed using transfer molded packaging techniques to produce low cost electrical/electronic functions in a single conventional package, such as a dual-in-line package (DIP). Inclusion of magnetic components such as transformers in hybrid integrated circuits has always presented a major challenge, because the transformer cores required usually have large cross-sectional areas. Such large cross-sectional areas are inconsistent with the need to provide circuit functions in small packages. Commonly assigned U.S. Pat. No. 4,103,267, by Olschewski and U.S. Pat. No. 4,847,986, by Meinel, indicate prior successful techniques for including miniature transformers in hybrid integrated circuits. However, the torroidal transformer components disclosed in the two foregoing references are not small enough to fit within a DIP package or the like. Also, a large number of wire bonding steps are required to complete the windings of these references, increasing the time and cost of manufacture. Also, the large number of wire bonded connections reduces reliability.
A prior art transformer 10 that is formed on a conventional printed circuit board, typically used for isolation amplifiers and DC-to-DC converters, is shown in FIG. 1. Spiral windings 12A and 13A are provided around holes 14A on one or more surfaces of printed circuit board 11. A ferrite core for transformer 10 includes an upper U-shaped section 18A and a lower U-shaped section 20A. The cross-sections of both sections 18A and 20A are square and uniform. The corresponding faces 20D on the legs 20C are attached by metal clips. The prior art transformer of FIG. 1, occupies a volume of approximately 3/4's of an inch on each side. The transformer 10 of FIG. 1 is inherently too large to be used in a hybrid integrated circuit. Its cross-section is uniform, probably because any reduction in the area of the magnetic flux path through the core of a transformer increases the amount of heat dissipated in the core and decreases transformer efficiency.
There are a variety of electronic products which are powered by rechargeable batteries. The functional operation of some electronic products, including some powered by rechargeable batteries, is made unreliable by the presence of electrical noise. In other products the presence of electrical noise causes sounds which are distracting to the user. Portable cellular telephones are an example of a noise-sensitive product. Some such products contain integral battery chargers that convert AC line voltage to low DC voltage levels at which conventional integrated circuit components can operate. It is important that the integral battery charger not introduce sufficient electrical noise to interfere with the reliable operation of the portable cellular telephone. There is an unmet need for a smaller, high efficiency, high frequency, low noise transformer that can be incorporated as part of an AC-to-DC or other circuit converter in a small hybrid integrated circuit package, such as a DIP package, or in a power connecter assembly.